1. Field of the Invention
The invention relates to a light source device. The invention relates especially to a light source device which is suited for a projector device using a DLP (digital light processor).
2. Description of Related Art
In a projector device using liquid crystals and DMD (digital micro mirror device), light radiated from a light source (discharge lamp) is focused using a reflector, a lens system and the like onto small elements for display of video information, and thus, they are illuminated. Furthermore, in this connection, the light transmitted or reflected by these small elements is radiated by an optical system, such as lenses or the like, onto a screen. In this connection, it is required that the discharge lamp is a point light source because the small elements are at most 1 inch, and moreover, because for smaller angular components of the incident light flux, the degree of utilization of light is high and the image contrast is improved.
For color projection of video information, there are a single leaf type and a triple leaf type. In the triple leaf type, after separation of the light radiated by the light source into three colors (Red)G(reen)B(lue), the light corresponding to the video information is transmitted or reflected by the respective display element, and afterwards, the three colors transmitted by the respective display element are combined and projected onto the screen.
On the other hand, for the single leaf type, the light radiated from the light source is radiated via a color wheel in which RGB regions are formed by division, reflected onto the DMD with which certain light is reflected, and radiated onto the screen. For the DMD, a few million small mirrors per pixel are located next to one another, the light projection being controlled by controlling the direction of the individual small mirrors.
In the case of the DMD type, a color image with R, G or B is projected on the screen in an extremely short time, due to this extremely short time, for the human eye, only composite color images are seen. In this DMD type, compared to the liquid crystal type, there is the advantage of a reduction in the size of the entire device, and its simplification, since the optical system is simple and three liquid crystal cells need not be used.
On the other hand, the light source of a projector device is a high pressure discharge lamp with high mercury vapor pressure since, by increasing the mercury vapor pressure, light in the visible wavelength range can be obtained with high efficiency. It is known that, in such a discharge lamp with a high mercury vapor pressure during operation, a projection is formed on the electrode tip. For example, Japanese patent application publication JP-A 2001-312997 and corresponding U.S. Pat. No. 6,545,430 disclose a technique in which the phenomenon of arc jump is effectively suppressed by stable focusing of the position of formation of the discharge arc on the projection. In this connection, it is specifically described that the growth of the projection is suppressed by given amounts of halogen and mercury, the given distance between the electrodes, the given operating frequency and the like.
In the case of a projector device of the DMD type, however, even in operation under the conditions under which the projection should be advantageously controlled, there arose the disadvantage of the arc becoming unstable as a result of the disappearance of this projection. This disadvantage occurs especially for an electrode on the side of the front opening of the oval reflector. It can be imagined that the reason for this is that the radiant light of the lamp is reflected by the color wheel and is emitted onto the electrode, by which this electrode is heated to an unexpectedly high temperature. In particular, since the color wheel is located more forward (lamp side) than the focal position of the oval reflector, the electrode is heated on the front opening side of the reflector.